Cho B = 1/3 +1/16 +1/19 +1/21 +1/61 +1/72 +1/83 +1/94. So sánh B

* Hướng dẫn giải

$\begin{array}{l}\text{B}=\text{B}=\frac{1}{3}+\left(\frac{1}{16}+\frac{1}{19}+\frac{1}{21}\right)+\left(\frac{1}{61}+\frac{1}{72}+\frac{1}{83}+\frac{1}{94}\right)\\ <\frac{1}{3}+\left(\frac{1}{15}+\frac{1}{15}+\frac{1}{15}\right)+\left(\frac{1}{60}+\frac{1}{60}+\frac{1}{60}+\frac{1}{60}\right)\\ <\frac{1}{3}+\frac{3}{15}+\frac{4}{60}=\frac{5}{15}+\frac{3}{15}+\frac{1}{15}=\frac{3}{5}\\ \Rightarrow B<\frac{3}{5}\end{array}$